Digital cameras or imagers sample the original light that bounces off a subject to create a digital image. A digital camera may use a shutter in combination with a sensor to determine an exposure time and acquire the proper amount of light to capture a good image. The proper amount of light may come from accumulating ambient light over time until the proper amount of light is obtained to accurately render the chrominance and luminance in the image.
In many digital CMOS imagers employing pixel arrays the optical integration time is controlled by the method known as “rolling shutter”. This approach utilizes at least two pointers, Reset and Read, that continuously move through the pixel array image frame from top to bottom by moving from row to row at predefined intervals. First, the Reset pointer 105 starts the integration for all pixels in the line. Some time later the Read pointer 110 reaches the same line and initiates signal readout. This is illustrated in FIG. 1, which depicts a pixel array 100. The distance in lines between the two pointers is referred to as shutter width 115. Shutter width multiplied by the line scan time gives the duration of the optical integration time. The exposure time can thus be adjusted by varying the shutter width.
The use of a “rolling shutter” approach allows an equal optical integration time for all pixels in an image frame to be achieved. However, when moving objects are being imaged, the rolling shutter can lead to artifacts. In the time it takes the shutter to move across the entire pixel array, the objects can change position, thereby distorting the resulting image. This problem can be overcome by using a global shutter in which all the pixels are exposed simultaneously. Unfortunately, this introduces a delay in reading out the data from the pixels. This delay can create problems when a very small time interval is needed between acquisition of successive images or when a fast frame rate is needed.